To test our hypothesis that the erythrocytic sedimentation rate (ESR) correlates well with clinical activity in inflammatory disease of the colon, but not of the small bowel, we stratified 49 Crohn's disease patients according to their anatomic involvement and then measured the correlations between ESR and clinical activity within each of these anatomical subgroups. For 18 patients with Crohn's disease involving primarily the colon, there was a trend toward a direct correlation between clinical score and ESR (p = 0.15). In the 14 patients with Crohn's disease limited to the colon, this direct correlation was even more pronounced and statistically significant (p less than 0.02). By contrast, an opposite trend was observed for patients with small bowel disease. For the 26 patients with disease involving predominantly the small bowel, as well as for the 22 with disease limited to small bowel, there were statistically significant inverse correlations between clinical score and ESR (p less than 0.04). This difference between the directions of the correlations for Crohn's colitis versus ileitis was statistically significant (p less than 0.05). This study provides further evidence for the importance of analyzing putative indications of disease activity separately for each of the protean forms in which Crohn's disease occurs.